Color in documents is the result of a combination of a limited set of colors over a small area, in densities selected to visually integrate to a desired color response. This is accomplished in many printing devices by reproducing separations of the image, where each separation provides varying density of a single primary color. When combined together with other separations, the result is a full color image.
In the digital printing and reproduction of documents, a separation is conveniently represented as a monochromatic bitmap, which may be described as an electronic image having a plurality of discrete signals (pixels) defined by position and density. In such a system, density is described as one of a number of possible states or levels. When more than two levels of density are used in the description of the image, the levels are often termed "gray", indicating that the density varies between a maximum and minimum, without reference to actual color. Most printing systems have the ability to reproduce an image with a small number of density levels, most likely two, although higher numbers of levels are possible. Common input devices including document scanners, digital cameras and the computer image generators, however, are capable of representing an image with a substantially larger number of gray levels or densities. A commonly selected number is 256 levels, although larger and smaller density ranges are possible. It is required that an image initially described at a large set of levels also be characterizable using a smaller set of levels, in a manner which captures the intent of the user. In digital reproduction of color documents this means that each of the color separations is reduced from the input number of levels to a smaller output number of levels. The multiple color separations are combined at printing to yield a final color print. Commonly, color documents are formed using cyan, magenta and yellow colorants or cyan, magenta, yellow and black colorants, although use of a larger number of primary colors, or alternative colorants, may be possible.
In the process of printing documents, the desired density of color over an area is commonly achieved by halftoning, where density variation of a color separation is achieved by placing greater or fewer "ON" pixels in a discrete area of a separation. In one halftoning method known as dithering or screening, over a given area comprising a number of gray separation pixels, a value representing the density of each pixel of an array of gray separation pixels is compared to one of a set of preselected thresholds (the thresholds are stored as a dither matrix and the repetitive pattern generated by this matrix is considered a halftone cell) as taught, for example, in U.S. Pat. No. 4,149,194 to Holladay or U.S. Pat. No. 5,223,953 to Williams which teaches an alternative hardware implementation of Holladay. The effect of such an arrangement is that, for an area where the image is gray, some of the thresholds within the dither matrix will be exceeded, i.e. the image pixel value is larger than the threshold value stored in the dither matrix for that same location, while others are not. In the binary case, the separation pixels or cell elements for which the thresholds are exceeded might be printed as a maximum colorant value, while the remaining separation pixels are allowed to remain as white (background), depending on the actual physical quantity described by the data. The described invention produces an output pattern that is periodic or quasi-periodic in the respective spatial coordinates, so as to implement accurate angle screens.
Dithering, however, creates problems in color document reproduction where the repeating pattern of a screen through the image, when superposed over similar repeating patterns for the remaining separations, causes moire or other artifacts. These problems arise particularly in printing systems with less than ideal registration between separations. It is also noted that moire remains a problem even for perfectly registered printing devices.
A color halftoning scheme using different angles for some or all of the color separations is common for applications that have slight misregistrations due to physical limitations. Accordingly, and with reference again to the Holladay patent (U.S. Pat. No. 4,194,194), the angle of the screen can be changed to generate similar screen patterns which do not strongly beat visually against each other, with the result that objectionable moire is reduced or eliminated. Particularly critical are the angles between the most prominent colors, cyan, magenta and black (if present). A common arrangement of rotated screen angles is 0.degree., 15.degree., 45.degree. and 75.degree. for yellow, cyan, black and magenta, respectively, in which case all separations are commonly halftoned using the same screen frequency, sometimes with the exception of yellow. However, objectionable patterning may still occur. Thus, the problem of moire is directly related to the inability of singleresolution output devices to achieve the accurate angles and frequencies necessary to reduce the periodic structure. While the problem may theoretically be reduced by using arbitrarily high resolutions (allowing closer approximation of the desired screen angles), such systems are more expensive and may not achieve the accurate angles so as completely eliminate moire. The present invention enables accurate angle screens without the necessity and impracticality of very high output resolutions.
The above described halftoning processes generate periodic halftone patterns. Other methods exist that generate non-periodic or quasi non-periodic structure. Examples for such methods are error diffusion and similar halftoning processes, stochastic screening and pulse density modulation. Yet another alternative to rotated screen halftoning is disclosed by Yang in the Xerox Disclosure Journal, Vol. 18, No. 4.
Some have proposed halftoning techniques that more or less directly emulate angularly oriented optical halftone screening functions, for example, U.S. Pat. No. 3,997,911 to Perriman et al. (Issued Dec. 14, 1976), U.S. Pat. No. 4,051,536 to Roetling (Issued Sep. 27, 1977) and U.S. Pat. No. 4,149,183 to Pellar et al. (Issued Apr. 10, 1979). Others have focused on modulating the size of the halftone dots that are written into tiled arrays of electronically generated halftone cells at a selected screen angle. See, for example: U.S. Pat. No. 3,688,033 to Hell et al. (Issued Aug. 29, 1972); U.S. Pat. No. 4,499,489 to Gall et al. (Issued Feb. 12, 1985); U.S. Pat. No. 4,680,645 to Dispoto et al. (Issued Jul. 14, 1987); and U.S. Pat. No. 5,258,849 to Tai et al. (Issued Nov. 2, 1993). Some have also described techniques applicable to electronic printing systems, for example, U.S. Pat. No. 4,868,587 to Loce et al. (Issued Sep. 19, 1989) and U.S. Pat. No. 4,185,304 to Holladay (Issued Jan. 22, 1980). In addition, U.S. Pat. No. 5,225,915 to Ciccone et al. illustrates that the addition of noise or enhancement of inherent noise can mask the structure moire. However, such schemes inherently alter the accuracy of the image.
In accordance with the present invention, there is provided a color document printing system for eliminating moire in printed output, the color document including a plurality of color separations wherein each separation is defined with a set of image signals representing optical density with m possible density levels, comprising: a printer, capable of rendering density with n density levels, adapted to print the color separations at distinct spatial resolutions, so that when the color separations are superposed on a substrate, a selected color is defined; a source of image signals describing the color document with a plurality of color separations, each image signal representing optical density as one of m levels for a discrete area of a color separation; a spatial resolution scaler, operatively connected to said source of image signals; a first halftone processor, operatively connected to said spatial resolution scaler, for reducing the number of levels m representing optical density in a subset of the color separations to a number of levels n representing optical density, said processor generating a first periodic pattern at a first spatial resolution; a second halftone processor, operatively connected to said spatial resolution scaler, for reducing the number of levels m representing optical density in at least one remaining color separation to a number of levels n representing optical density, said processor generating a second periodic pattern at a second spatial resolution; and a video processor directing the distinct spatial resolution signals processed at each halftone processor to said printer to print the processed color image.
In accordance with another aspect of the present invention, there is provided a method for preparing a color document for moire-free printing, the color document including a plurality of color separations wherein each separation is defined with a set of image signals representing optical density with m possible density levels, while a printer is capable of rendering density with n density levels, comprising: receiving a set of image signals describing the color document with a plurality of color separations, each image signal representing optical density as one of m levels for a discrete area of a color separation; spatially scaling the image signals of a first color separation; halftoning the m level image signals of the first color separation to n level image signals representing optical density in a manner generating a first periodic pattern having a fixed frequency and a distinct first angle for the first separation; halftoning the m level image signals of a second color separation to n level image signals representing optical density in a manner generating a second periodic pattern having the fixed frequency and a distinct second angle for the second separation; and directing the first and second periodic patterns to said printer to print the processed color image.